there was no reason for it except to cause his victims as much pain as possible
which is beyond cruel and sadistically evil. he KNOWS revenge wont bring sasuke peace or take away the pain. he KNOWS sasuke already had a burning hate for him for killing his whole family. and he knows his clan do not take loss well and the likely result of leaving sasuke alive is he loses his mind and uses that "get as strong as possible" thing to also get as evil and unstoppable as possible. but itachi didnt care how sauske used his newfound power did he? he didnt care he took away sasuke's mental fortitude to wield power properly. all he cared about was sasuke helping him kill himself. consequences didnt matter.